1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copier or a printer, that uses an electrophotographic method or an electrostatic recording method.
2. Description of the Related Art
Among image forming apparatuses, so-called laser printers that use an electrophotographic method, in which a photosensitive member is charged and an image is exposed using a laser beam and developed, are known. Such laser printers have advantages of high image quality, high speed, and the like and are in widespread use as, for example, output apparatuses such as copiers or ordinary printers. A developing device provided in such an image forming apparatus uses a one-component developing material mainly composed of a magnetic toner or a two-component developing material mainly composed of a non-magnetic toner and a magnetic carrier. In particular, most developing devices of image forming apparatuses for forming full-color or multi-color images use a two-component developing material from the standpoint of the tint of images.
In recent years, such image forming apparatuses have been increasingly used in the quick printing market, which is called POD (print on demand), and to keep up with this trend, there is a growing demand for further increases in image quality and stability. One of the items that are regarded as important in achieving the increase in stability is stabilization of density tone characteristics. Since an image forming apparatus is highly sensitive especially to environmental fluctuation, durability fluctuation, and the like in terms of a change in output density, it is necessary to keep the density tone of images in a proper state at all times.
A method for stabilizing the density tone is auto tone correction processing disclosed in Japanese Patent Laid-Open No. 2000-238341. In an example of such a method, a tone pattern image is formed on a recording material, an optical density of this image is read, a γLUT (look up table) is thus created so that desired tone characteristics can be obtained, and tone control is performed with this table. Auto tone correction processing makes it possible to obtain stable density tone characteristics regardless of environmental fluctuation, durability fluctuation, or the like.
Another method for stabilizing the density tone is a method called patch detection ATR (auto toner replenisher) control disclosed in Japanese Patent Laid-Open No. 10-039608, which stabilizes the charge characteristics of a toner. This method stabilizes the charge characteristics of a toner in the case where a two-component developing material is used. A reference image pattern (a patch image) for image density detection is formed on an electrophotographic photosensitive member (a photosensitive member), the patch image is detected by an image density sensor, the amount of toner to be supplied is controlled so that the detection value becomes a predetermined value, and thus the charge characteristics of the toner are stabilized.
However, a problem with auto tone correction processing is that if there is a change in the state, such as the toner charge characteristics, of the developing material after auto tone correction processing, in some cases, optimum density tone characteristics can no longer be maintained. For example, an environmental fluctuation or a time period for which the developing material is left to stand may cause a situation in which when desired toner charge characteristics are not obtained and therefore auto tone correction processing is performed, once the toner charge characteristics return to a desired value by patch detection ATR control after the auto tone correction processing, the density tone characteristics significantly deviate.
As a countermeasure to this problem, Japanese Patent Laid-Open No. 2000-238341 proposes a method for stabilizing the density tone characteristics after auto tone correction processing by changing the density tone characteristics using the density of a patch image on an image carrier. However, with this method, since the density of the patch image on the image carrier is the density before transfer and fixing steps, the accuracy may be inferior to that of auto tone correction processing, which uses the density of the image formed on the recording material, and therefore this method cannot sufficiently meet the recent demand for increased stability. Furthermore, since it is necessary to form a multi-tone patch image in order to enhance the effect of this method, downtime during image formation increases and the amount of toner used also increases, and therefore desired cost effectiveness cannot be achieved.
Meanwhile, it seems that the tone characteristics can be maintained if a target value of patch detection ATR control is changed so that the toner charge characteristics after auto tone correction processing are maintained. However, in this state, a malfunction may occur because the developing material is not in a desired state, that is, for example, the toner charge amount is low. Specifically, it is expected that deterioration in image quality (a decrease in graininess (roughness), background fog, white spots due to transfer defects, and the like), scattering of toner, and the like will occur.